The present invention relates to corrosion monitoring and measuring and, more particularly, to a compact system for detecting localized corrosion at various positions on civil engineering structural members, especially of steel material, in corresponding microenvironments.
The useful life of a structural member used in a corrosive microenvironment, which may differ from the environment extant only a short distance away, is necessarily limited by corrosion. It is economically desirable, however, that such a structural member be used as long as possible. Therefore, there is a demand for monitoring the status of corrosion of the structural member over long periods of time for the purpose of estimating the member's residual service life. To satisfy that demand, a corrosion monitor system should accurately measure the actual degree of corrosion at various positions on the structural member as it occurs in the microenvironments of use.
Unfortunately, one of the most difficult aspects of corrosion studies is the development of a system by which the resistance of different metals to certain corroding media and conditions can be evaluated. Because many variables are involved in the corrosion process, it is difficult to devise a system which will yield results commensurable with service conditions. Tests are often set up in the laboratory to show the effects of corrosion. When structures are placed in service under apparently identical conditions, however, the behavior may be quite different. In many instances, therefore, it is necessary to test structural materials in the service environment over extended time periods.
An observation frequently made concerning corrosion of structures, especially steel structures, is that the most serious attack on the structure occurs in areas where ionic contaminants (particularly chlorides), debris, and corrosion products have accumulated. In the case of weathering steels, severe localized corrosion may also occur. (A. Raman, Degradation of Metals in the Atmosphere, ASTM STP 965, S.W. Dean & T.S. Lee, Eds., 1988, pp. 16-29.) That problem has been studied in automobile bodies, where debris and road dirt can collect in crevices, wheel wells, and other entrapment areas. This accumulation is referred to as "poultice," and can cause severe corrosion of the automobile body, although the chemistry which occurs in the poultice is complex and varies with geographic location and whether the material is continually wet or has wet/dry cycles. (R.C. Turcotte, T.C. Comeau & R. Baboian, Materials Degradation Caused by Acid Rain, ACS Symposium Series 318, 1986, pp. 200-12.)
The same effects should occur for larger steel structures. The effects of ionic contaminants, particularly chlorides, on the "critical humidity" necessary for rusting of steels to occur have been well documented. (K.A. Chandler, Brit. Corros. J., 1, 264-66 (1966); P.R. Vassie, Brit. Corros. J., 22, 37-44 (1987).) In addition, chlorides accelerate corrosion. Any debris on the steel that holds moisture exacerbates the effect of ionic contaminants.
Despite the known influence of debris accumulation on corrosion, the corrosion monitors that are presently known do not include provisions for that effect. Devices exist that measure polarization curves (J.A. Gonzalez, E. Otereo, C. Cabans & J.M. Bastidas, Brit. Corros. J., 19, 89-94 (1984)), impedance (S. Haruyama & T. Tsuru, Electrical Corrosion Testing, ASTM STP 727, F. Mansfeld & U. Bertocci, Eds., 1981, pp. 167-86; S. Ito et al., Nippon Steel Technical Report No. 32, Jan. 1987), or time of wetness (F. Mansfeld et al., Atmospheric Corrosion of Metals, S.W. Dean & E.C. Rhea, Eds., ASTM STP 767, 1982, pp. 309-38; V. Kucera & J. Gullman, Electrochemical Corrosion Testing, F. Mansfeld & U. Bertocci, Eds., ASTM STP 727, 1981, pp. 238-55), but these devices do not include the effect of poultice on the output. A related investigation monitored marine corrosion by comparing the output of several galvanic cells in laboratory-simulated environments. (V.S. Agarwala, Atmospheric Corrosion, W.H. Ailor, Ed., 1982, pp. 183-92.) No attempt was made, however, to accumulate debris to simulate what might occur on a bridge structure.
The presently known corrosion monitors also include the use of galvanic cells to predict the rate at which corrosion will occur in a steel structure situated in a specific environment. Moreover, devices are capable of remote monitoring so that the progress of corrosion can more easily be followed over a long period of time. Some monitors operate in the same environment as the structures they intend to evaluate. None of the corrosion monitors collect corrosion products and environmental debris, however, even though the effects of those materials will influence the amount of corrosion which actually occurs.